The reconstruction of bones, for example of the skull, has been an ongoing intensive research. With reference to the skull reconstruction, whereas several reports focus on the reconstruction of large and complex-shaped cranial defects comparatively little has been reported about restoration of small but cosmetically undesirable osseous gaps in trephined burr holes. Trephination burr holes often result in small but undesirable scalp and skin depression. Subdural hematoma is a common problem especially if patient has head injuries related to accidents or due to blood clogging in the brain as a result of stroke. It is usually treated by burr hole drainage or irrigation. The trephined burr hole procedure involves drilling a hole typically 14 to 19 mm in diameter on to the patient's skull.
Various bone grafts or bone substitute materials have been used to fill those defects which normally do not enable the bone to regenerate and recover the defect. Tessier (Tessier 1982) has reported the use of split calvarial autologous grafts to bridge or fill defects. This technique represents a cheap and straight forward approach however sometimes the primary incision has to be extended in order to harvest the graft from the surrounding calvarial bone. However, there are problems associated with the use of bone tissue grafts. If the patient's own bone is used as a graft, a surgeon must perform an additional, traumatic operation to take the bone sample. If the bone graft is taken from another person or animal bone is used, viral contaminations or immunological problems are possible, even if the graft is treated to make it compatible with the patient's tissue.
Another possibility using autologous graft material is to collect the bone dust during the craniotomy procedure and mix it with a hydrogel like fibrin glue and use that paste to fill out the defect after the procedure (Matsumoto, 1998).
Cranioplastic materials based on metal have been extensively used in the form of titanium plates and meshes. The high biocompatibility and mechanical strength in combination with the easy handling and accurate fixation thus might justify the relatively high costs (Broaddus, 2002). Silastic, a commonly used biomaterial in medicine is also used as burr hole cover however controversy discussed in terms of its biocompatibility as reports indicate the formation of foreign body reaction due to pathologic tissue response to its elastomers (Winkler 2000).
In recent years there is a move towards osteoinductive biomaterials and implants which allow the ingrowth of bone tissue and therefore better integration of the implants. The trend using bioresorbable materials and tissue engineering has resulted in protheses which are eventually replaced by autologous bone (Habal 1999, Stendel 2001, Schantz 2003a, b). Kobayashi et al (1987) have designed and fabricated various alumina ceramic implants to reconstruct trephination burr holes and to prevent postoperative dents in the skin. Ceramic implants based on hydroxyapaptite are increasing popular due to their mechanical properties, osteoinductive and integrative characteristics (Yamashina, 1989, 1993, Miake, 2000), Yamashina has designed hydroxyapatite plates which are domed and elliptic in shape so that they fit the convexity of the occipital region. The author has also designed HA-buttons to fit burr hole defects as well as apatite granules for linear skull defects. A specially designed “key-hole button” based on hydroxyapatite was designed by Koyama et al (2000) for trephination defects.
Various surgical approaches and implantable device have been developed especially for the treatment of acute or chronic subdural hematomas associated burr hole defects. In these cases it is often desirable to place a shunt or catheter to monitor or drain intra or pericranial fluid and parallely to monitor pressure characteristics. Emonds and Hassler have developed a hollow screw which allows placement of a catheder (1999) whereas Dujovny et al (2002) designed a burr hole cover for a hydrocephalus shunt drainage based on titanium consisting of a circular plate with five attached flaps for screws and a key hole like opening.
U.S. Pat. No. 6,350,284 ('284) describes a bioabsorbable cranial implant consisting of a rigid plate and a fibrous web layer containing pores between 30 and 1000 μm in diameter. This implant, however, requires to be fixed to the bone by means for attachment, for example, sutures, tacks, or screws, and it is therefore not practical.